1. Field of the Invention
The present invention relates to an electro-static discharge (ESD) protection device to protect an electronic circuit or an electronic component from a surge. In particular, the present invention relates to an ESD protection device having improved resistance to breakdown caused by heat or shock during discharge, and a method for producing the same.
2. Description of the Related Art
In general, electrical devices may be damaged when subjected to an electrostatic discharge such as a surge. In such a case, electro-static discharge (ESD) protection devices may be used in order to protect circuit elements from an electrostatic discharge. ESD protection devices are devices in which an excessive voltage is conducted from a signal line to a ground line by utilizing a discharge phenomenon in order to protect circuit elements from static electricity.
A known ESD protection device includes insulating ceramic layers, a pair of outer electrodes provided on outer surfaces of the insulating ceramic layers, inner electrodes that are provided inside the insulating ceramic layers and that are electrically connected to the outer electrodes, a discharge space provided in the insulating ceramic layers (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-043954).
FIG. 1 is a perspective view of an ESD protection device 10 disclosed in Japanese Unexamined Patent Application Publication No. 2001-043954. FIG. 2 is an exploded perspective view of the ESD protection device 10. FIG. 3 is a cross-sectional view taken along line A-A in FIG. 1. As illustrated in FIG. 2, the ESD protection device 10 is a laminate of insulating ceramic sheets 2, an insulating ceramic sheet 3 having an opening 5, and an insulating ceramic sheet 4 having an opening 5 and inner electrodes 6. In the laminated state, as illustrated in FIG. 3, the pair of inner electrodes 6 is arranged inside insulating ceramic layers 7 so as to face each other, and facing portions of the inner electrodes 6 are exposed in a discharge space 8. Outer electrodes 1 that are electrically connected to the inner electrodes 6 are formed on end surfaces of the laminate. As illustrated in an ESD protection device 10a of FIG. 4, a portion of each of the inner electrodes 6 and an insulating ceramic layer 7 may be separated from each other in the discharge space 8.
When the ESD protection device 10 or 10a is mounted on a printed wiring board P, a gap d is formed between the laminate and the printed wiring board P.
In the ESD protection device having the existing structure disclosed in Japanese Unexamined Patent Application Publication No. 2001-043954, it is difficult to dissipate heat generated near the facing portions of discharge electrodes by a surge current flowing during the operation of ESD protection. Accordingly, when the surge current repeatedly flows within a short time, the calorific value in the facing portions of the discharge electrodes significantly increases, and thus melting of the electrodes or vitrification of the ceramic material may occur near the facing portions of the discharge electrodes. Cracks (microcracks) and breaking may also be caused by a thermal stress. These phenomena may cause a short-circuit defect.